This invention relates to a fuel injection pumping apparatus for supplying fuel to a compression ignition engine and of the kind comprising a low pressure pump which supplies fuel to a high pressure pump, the output pressure of the low pressure pump varying in accordance with the speed at which the apparatus is driven, a fuel pressure operable device for varying the timing of fuel delivery by the high pressure pump and means for controlling the quantity of fuel which is supplied by the high pressure pump.
It is known that the timing of delivery of fuel to a compression ignition engine must be carefully controlled in order to avoid the emission of noxious exhaust gases.
A known apparatus of the aforesaid kind is seen in GB 2174515B, in which fuel from the low pressure pump flows through a first orifice and then through a second orifice with the pressure developed intermediate the orifices being applied to the pressure operable device. Downstream of the second orifice is a variable orifice through which the fuel can flow to a drain and the degree of restriction offered by the variable orifice is arranged to vary in accordance with the amount of fuel delivered by the high pressure pump, the degree of restriction increasing as the quantity of fuel supplied by the high pressure pump is decreased. Arranged in parallel with the aforesaid variable orifice is a bypass valve which has a valve member responsive to the pressure intermediate the first and second orifices and moving with increasing pressure against the action of a spring to progressively open a bypass passage to drain. The extent of movement of the valve member is limited by an adjustable stop to control the maximum effective size of the bypass passage.
The setting of the adjustable stop is critical for the correct functioning of the apparatus and in some instances it has been found to be very difficult to adjust the setting of the stop.